fix: (Bug) correct resultType when storing instant query results in cache

[kavirajk]

What this PR does / why we need it:
tldr; Previously the results cache for instant metric query is stored as wrong type TypeMatrix so some of the results were ignored during final computation. This PR fixes the correct type TypeVector

Before

]-bash-5.2$ ./cmd/logcli/logcli instant-query 'sum(rate({job="varlogs"}[1h]))' -q
[
  {
    "metric": {},
    "value": [
      1711117293.063,
      "0.5336111111111111"
    ]
  }
]-bash-5.2$ ./cmd/logcli/logcli instant-query 'sum(rate({job="varlogs"}[1h]))' -q
[
  {
    "metric": {},
    "value": [
      1711117295.905,
      "0.19944444444444445"
    ]
  }
]-bash-5.2$

First query will miss the cache and store it in the cache after downstream requests. Second request fetch from the cache. NOTE both are are same (bug)

After

-bash-5.2$ ./cmd/logcli/logcli instant-query 'sum(rate({job="varlogs"}[1h]))' -q
[
  {
    "metric": {},
    "value": [
      1711117478.337,
      "0.5011111111111111"
    ]
  }
]-bash-5.2$
-bash-5.2$ ./cmd/logcli/logcli instant-query 'sum(rate({job="varlogs"}[1h]))' -q
[
  {
    "metric": {},
    "value": [
      1711117484.699,
      "0.5011111111111111"
    ]
  }
]-bash-5.2$

NOTE: both first (without cache) and second (with cache) have same results

Detail explanation

The bug is the assumption of existing ResultCache that it always works with range queries everytime it has to store LokiPromResponse in the cache. But in reality we share the same response for both instant and range queries. With adding support for instant queries recently.

But I failed to update the case where ResultsCache whenever trying to merge the multiple cache entries (we call Extent) it create new bigger response type (LokiPromResponse) with resultType set to matrix (assuming only range queries uses it)

So when the cache is hit and these values are returned for computing the merged responses, it considers only samples with resultType vector (because it's instant query). That's the reason, when original query split into multiple subqueries and some of those subqueries even though hit cache, it's value is ignored in the final computation. Hence wrong results.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
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[sandeepsukhani]

LGTM